1. Field of the Invention
The present invention relates to an error propagation control method based on decision feedback equalization (DFE) including multi-level decision feedback equalization (MDFE), and to a recording/reproducing device, such as a magnetic disk device, which makes use of this method.
2. Description of the Related Art
Recently, the recording density of magnetic disk devices and magnetic tape recording devices are dramatically increasing. It is expected that the surface recording density of magnetic disk devices will reach 10-20 Gb/in2.
The increase in surface recording density means an increase in transfer speeds. The increase in transfer speeds induces an increase in the recording frequency if the same recording encoding and signal processing method are used, and writing heads have reached its recording limit. Because of this, an improvement in heads and an improvement in recording media are in-progress. Efforts to develop a signal processing method are also being made.
For signal processing, a decrease in the space between bits in particular, and the inter-symbol interference caused by this, deteriorates the signal-to-noise ratio (SNR). A conventional method of using run length limited (RLL) codes, which involves peak detection and read equalization, is insufficient for high densities.
Therefore John G. Kenney and others proposed multi-level decision feedback equalization (MDFE) technology as a new processing method (IEEE Transactions on Magnetics, Vol. 29, NO. Jul. 3, 1993: xe2x80x9cMulti-level Decision Feedback Equalization for Saturation Recordingxe2x80x9d).
FIG. 14 is a block diagram of a configuration of a magnetic disk device, which makes use of the decision feedback equalization (DFE) method, including multi-level decision feedback equalization (MDFE), particularly of a signal processing system.
In FIG. 14, an input NRZ signal, which is composed of writing target xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, is converted by a (1,7) RLL encoder into a (1, 7) RLL code, where the number of xe2x80x9c0xe2x80x9d s between bit 1 and bit 1 is at least 1 and at most 7, that is, the number of continuous alternations is 1 and the maximum continuance of same polarity codes is 8.
The output of the (1, 7) RLL encoder 1 has a value of xc2x11 sampled at a timing of 1/T. With respect to the output of the (1, 7) RLL encoder 1, the write FF circuit 2 calculates (1/1xe2x88x92D)mod2 sends the calculation result to the write/read head 3 and writes it to a disk.
FIG. 15 shows an example of written and read out waveforms, which are written to and read out from the disk by the above mentioned write/read head 3. The write waveform, which is output from the write FF circuit 2, is as shown in FIG. 15 (A).
In FIG. 14, the data written to the disk is read by the write/read head 3, and is amplified for reproduction to a specific level through a head pre-amplifier circuit 4 and an AGC amplifier 5.
FIG. 15 (B) shows a signal waveform read by the write/read head 3. The output of the AGC amplifier 5 is input to a forward filter 6 of an MDFE circuit 10, and is output as the waveform shown in FIG. 15 (C). In other words, [the output of the AGC amplifier 5] is converted to a ternary signal (xe2x88x922, 0, +1).
The feedback filter 7 feeds back the sum of the outputs of a detector 9 multiplied by a predetermined coefficient to the input side. When an input pulse is generated, the output of the feedback filter 7 changes the polarity, making it the reverse of the pulse polarity (FIG. 15 (D)). In other words, the feedback filter 7 assumes that the polarity of the reproduction signal pulses alternates. Therefore, the output polarity of the feedback filter 7 is usually the opposite of the polarity of the expected input to the forward filter 6.
A difference circuit 8 determines the difference between the output of the forward filter 6 (FIG. 15 (C)) and the output of the feedback filter 7 (FIG. 15 (D)). As a result, the output of the difference circuit 8 has a waveform centered around the xe2x80x9c0xe2x80x9d level, as shown in FIG. 15 (E). Here, the difference circuit 8 can be configured such that the sum of the output of the forward filter 6 and the output of the feedback filter 7 is determined.
The output of the difference circuit 8 is then subjected to binary decision by a detector 9. The output of the detector 9 is a binary coded sequence, as shown in FIG. 15 (F), and is set in a quaternary state by 1-bit convolution by the MDFE circuit 10, therefore, [the output of the detector 9] deviates 1 bit from the write data code string (FIG. 15 (A)).
A (1xe2x88x92D)mod2 circuit 11 calculates (1xe2x88x92D)mod2, which is the inverse of the processing of the write FF circuit 2, and decodes the (1, 7) RLL codes by a (1, 7) decoder 12. In this way the read signal is reproduced.
According to the structure of the magnetic disk device which makes use of the decision feedback equalization (DFE), including multi-level decision feedback equalization (MDFE), the decision result on the read signal by the detector 9 is fed back.
Also, the tap coefficient of the feedback filter 7 is set on the condition that the polarity of the head reproduction signal to be input alternates.
The inventors recognized that errors continue and propagate when the above condition is not satisfied in a system which makes use of the decision feedback equalization (DFE), including multi-level decision feedback equalization (MDFE), and further studied the conditions under which such errors propagate.
With the foregoing in view, it is an object of the present invention to provide an error propagation control method and a magnetic recording/reproducing device which makes use of this method for use in systems based on decision feedback equalization (DFE), including multi-level decision feedback equalization (MDFE).
The basic constitution of the decision feedback equalization (DFE) method, including multi-level decision feedback equalization (MDFE), for achieving the above mentioned object of the present invention, and the magnetic recording/reproducing device which makes use of this method, involves outputting the difference between or the sum of an input signal and a feedback signal and detecting the level [of the input signal] with respect to the difference or sum signal output based on the slice level.
Then the error propagation of the input signal is decided, and based on the result of the error propagation decision, a predetermined offset is added to the above mentioned slice level.
Also, based on the result of the error propagation decision, a predetermined DC offset is added to the DC level of a signal which level is detected with reference to the above mentioned slice level.
Or, based on the result of the above mentioned error propagation decision, a sign of the tap of the feedback filter, which feeds back the above mentioned detected output to the above mentioned input signal side, is reversed.
An aspect of the present invention is characterized in that the above mentioned error propagation of the input signal is decided based on the output when the signal level is detected with reference to the above mentioned slice level.
Another aspect of the present invention is characterized in that the above mentioned error propagation of the input signal is decided based on the thermal asperity detection signal.
Another aspect of the present invention is characterized in that the input signal to be the target of the above mentioned decision of the error propagation is data, or a preamble, or a synchronous word of the data.
Another aspect of the present invention is characterized in that when the above mentioned input signal for which the difference or sum with the above mentioned feedback signal is determined is encoded with (d, k) run length limited encoding, where the minimum magnetization reversal interval is d and the maximum magnetization reversal interval is k, the state of error propagation to be detected is a d constraint or a k constraint violation state.
Another aspect of the present invention is characterized in that when the above mentioned input signal for which difference or sum with the above mentioned feedback signal is determined is encoded with (1, 7) run length limited encoding, where the minimum magnetization reversal interval is 1 and the maximum magnetization reversal interval is 7, the above mentioned error propagation state is detected by detecting nine samples or more of continuous same polarity signals or by detecting two samples or more of continuous polarity alternations.
Another aspect of the present invention is characterized in that when the above mentioned input signal for which a difference or sum with the above mentioned feedback signal is determined is encoded with (d, k) encoding where the minimum magnetization reversal interval is d and the maximum magnetization reversal interval is k, the above mentioned error propagation state is detected when two samples or more of continuous polar alternations are generated two or more times, or k+2 samples or more of same polarity signals continue.
Another aspect of the present invention is characterized in that when the above mentioned input signal for which a difference or sum with the above mentioned feedback signal is determined is encoded with (d, k) encoding where the minimum magnetization reversal interval is d and the maximum magnetization reversal interval is k, the above mentioned input signal has a plurality of (d, k) limitations, and the decision of violating either a d constraint or a k constraint corresponding to the respective (d, k) limitation can be selected.